Yolk-Shell Construction of Na3 V2 (PO4 )2 F3 with Copper Substitution Microsphere as High-Rate and Long-Cycling Cathode Materials for Sodium-Ion Batteries.

Na3 V2 (PO4 )2 F3 (NVPF) is emerging as a promising cathode material for high-voltage sodium-ion batteries. Whereas, the inferior intrinsic electrical conductivity leading to poor rate performance and cycling stability. To address this issue, a strategy of synthesizing unique yolk-shell structured NVPF with copper substitution via spray drying method is proposed. Besides, the synergistic modulation of both crystalline structure and interfacial properties results in significantly enhanced intrinsic and interfacial conductivity of NVPF. The optimized yolk-shell structured cathode materials can possess a high capacity of 117.4 mAh g-1 at 0.1 C, and remains a high-capacity retention of 91.3% after 5000 cycles. A detailed investigation of kinetic properties combined with in situ XRD technology and DFT calculations, has been implemented, particularly with regard to electron conduction and sodium ion diffusion. Consequently, the yolk-shell structured composition of Na3 V1.94 Cu0.06 (PO4 )2 F3 with nitrogen-modified carbon coating layer shows the lowest polarization potential because of the effectively enhanced electronic conductivity and Na+ diffusion process in the bulk phase. The robust electrochemical performance suggests that developing the unique yolk-shell structure with the collaboration of interface and bulk crystal properties is a favorable strategy to design cathode material with a high performance for sodium-ion batteries.